1. Field of the Invention
The invention relates to a unit for generating signal pulses, comprising a first, pulsed laser for supplying a pulse series having a pulse period T and a wavelength .lambda..sub.1, and a modulation unit for modulating the laser in accordance with a data signal.
The invention also relates to a multiplex transmission system and a transmitter suitable for use in such a system.
2. Discussion of the Related Art
In optical transmission systems for information transport over large distances, an optical transmitter provided with a laser is used for converting a digital electric signal into optical pulses. The laser radiation is modulated in accordance with the signals to be transmitted. In this way a series of optical pulses is formed which can be transported through an optical fiber to an optical receiver in which it is converted into a digital electric signal.
For such transmission systems the aim is, inter alia a larger transmission distance. However, in the optical fiber the transmitted pulses are attenuated due to damping. Generally, the width of the propagating pulses increases as the length of the optical fiber increases. This pulse Widening is the result of the fact that the travel time in the optical fiber is different for radiation of different wavelengths. This phenomenon is referred to as dispersion. A pulse transmitted by the optical transmitter will generally comprise components of different Wavelengths which, due to dispersion, reach the receiver at different instants.
One way of reducing the detrimental effects of pulse widening and damping is to use solitons as signal carrying pulses. Such radiation pulses have such a radiation distribution with respect to time and such a power that pulse narrowing occurs due to a non-linear effect in the optical fiber. If the power of the optical pulses assumes a value within a given interval, it is possible that the effects of pulse widening and pulse narrowing cancel each other.
One of the possibilities of generating solitons is referred to as gain-switching of a diode laser. In this method a short current pulse is applied to a diode laser which in its turn transmits a short optical pulse having a relatively short pulse length, for example of the order of 30 to 40 psec. By passing the optical pulses thus obtained through an optical fiber having a given length and a normal dispersion, its pulse length can be reduced to 15 to 20 psec. These pulses are suitable to be propagated as solitons through an optical fiber. If such a pulse series is to be modulated with a signal to be transmitted, the following problem occurs. To obtain said short optical pulses, the modulation of the current through the laser should be controlled in such a way that each optical pulse is generated by only the relaxation oscillation of the laser. The laser oscillation is determined by the charge carder density and the photon density in the laser medium. The supply of a data signal to the laser means that different current patterns are applied to the laser. These current patterns cause different charge carder densities in the laser medium so that the shape of the optical pulses will start to vary. It has been found that there is a large variation, not only in pulse shape but also in the instant of pulse formation if solitons obtained by gain-switching are to be modulated with data. Jitter is produced so that the pulse position will not be sharply defined with respect to the pulse period.
Another possibility of generating solitons is referred to as mode-locking. In this method a diode laser having an anti-reflective coating is placed on one of the exit faces of an external resonant cavity. Subsequently, the current through the laser is modulated at a repetition frequency which is suitable for the circulation time of the external resonant cavity or for a higher harmonic of this frequency. Then the laser is controlled by the radiation which has been fed back, and after the radiation has circulated several times, the laser will supply short pulses whose length and spectral width are Fourier-limited. A data pulse series to be transmitted can be obtained by modulating, for example the pulse series obtained via mode-locking with the aid of an external modulator arranged outside the resonant cavity and controlled by the data signal to be transmitted, as described, for example in the article "Monolithic semiconductor soliton transmitter" by P. B. Hansen et al. in OFC '94 Technical Digest, pp. 74-75. The diode laser whose pulse series is obtained by mode-locking and the modulator are integrated in a single element in this case.
The drawback of an,external modulator is that it causes extra optical losses and that no sufficient distinction between a digital "1" and a digital "0" can be obtained in the optical signal at the desired high switching rates.
The direct modulation, with a data signal, of pulses which are obtained through mode-locking and may propagate as solitons, i.e. modulation, with the data signal, of the electric current through the laser, is neither possible because then the process of generating pulses suitable for forming solitons is disturbed. In fact, this process requires a continuous feedback in the external resonant cavity. Whenever the current through the laser is interrupted in conformity with the electric data signal, this continuous feedback is no longer realised. For this reason the repetition time of the modulator with which the data signal to be transmitted is applied to the pulse series of the diode laser should be adapted to the length of the resonant cavity for the purpose of modulation. This results in considerably strict tolerances in the manufacture of such units.